Detector for x-ray fluorescence

ABSTRACT

Disclosed herein is a detector, comprising: a plurality of pixels, each pixel configured to count numbers of X-ray photons incident thereon whose energy falls in a plurality of bins, within a period of time; an X-ray absorption layer; wherein the X-ray absorption layer comprises an electrical contact within each of the pixels, and a focusing electrode surrounding the electrical contact and configured to direct to the electrical contact charge carriers generated by an X-ray photon incident within confines of the focusing electrodes; and wherein the detector is configured to add the numbers of X-ray photons for the bins of the same energy range counted by all the pixels.

TECHNICAL FIELD

The disclosure herein relates to a detector suitable for X-rayfluorescence.

BACKGROUND

X-ray fluorescence (XRF) is the emission of characteristic fluorescentX-rays from a material that has been excited by, for example, exposureto high-energy X-rays or gamma rays. An electron on an inner orbital ofan atom may be ejected, leaving a vacancy on the inner orbital, if theatom is exposed to X-rays or gamma rays with photon energy greater thanthe ionization potential of the electron. When an electron on an outerorbital of the atom relaxes to fill the vacancy on the inner orbital, anX-ray (fluorescent X-ray or secondary X-ray) is emitted. The emittedX-ray has a photon energy equal the energy difference between the outerorbital and inner orbital electrons.

For a given atom, the number of possible relaxations is limited. Asshown in FIG. 1A, when an electron on the L orbital relaxes to fill avacancy on the K orbital (L→K), the fluorescent X-ray is called Kα. Thefluorescent X-ray from M→K relaxation is called Kβ. As shown in FIG. 1B,the fluorescent X-ray from M→L relaxation is called La, and so on.

Analyzing the fluorescent X-ray spectrum can identify the elements in asample because each element has orbitals of characteristic energy. Thefluorescent X-ray can be analyzed either by sorting the energies of thephotons (energy-dispersive analysis) or by separating the wavelengths ofthe fluorescent X-ray (wavelength-dispersive analysis). The intensity ofeach characteristic energy peak is directly related to the amount ofeach element in the sample.

Proportional counters or various types of solid-state detectors (PINdiode, Si(Li), Ge(Li), Silicon Drift Detector SDD) may be used in energydispersive analysis. These detectors are based on the same principle: anincoming X-ray photon ionizes a large number of detector atoms with theamount of charge carriers produced being proportional to the energy ofthe incoming X-ray photon. The charge carriers are collected and countedto determine the energy of the incoming X-ray photon and the processrepeats itself for the next incoming X-ray photon. After detection ofmany X-ray photons, a spectrum may be compiled by counting the number ofX-ray photons as a function of their energy. The speed of thesedetectors is limited because the charge carriers generated by oneincoming X-ray photon must be collected before the next incoming X-rayhits the detector.

Wavelength dispersive analysis typically uses a photomultiplier. TheX-ray photons of a single wavelength are selected from the incomingX-ray a monochromator and are passed into the photomultiplier. Thephotomultiplier counts individual X-ray photons as they pass through.The counter is a chamber containing a gas that is ionizable by X-rayphotons. A central electrode is charged at (typically)+1700 V withrespect to the conducting chamber walls, and each X-ray photon triggersa pulse-like cascade of current across this field. The signal isamplified and transformed into an accumulating digital count. Thesecounts are used to determine the intensity of the X-ray at the singlewavelength selected.

SUMMARY

Disclosed herein is a detector, comprising: a plurality of pixels, eachpixel configured to count numbers of X-ray photons incident thereonwhose energy falls in a plurality of bins, within a period of time; anX-ray absorption layer; wherein the X-ray absorption layer comprises anelectrical contact within each of the pixels, and a focusing electrodesurrounding the electrical contact and configured to direct to theelectrical contact charge carriers generated by an X-ray photon incidentwithin confines of the focusing electrodes; and wherein the detector isconfigured to add the numbers of X-ray photons for the bins of the sameenergy range counted by all the pixels.

According to an embodiment, the focusing electrode comprisespolysilicon, a metal or a metal alloy.

According to an embodiment, the focusing electrode is not in directcontact with the electrical contacts.

According to an embodiment, the focusing electrode comprises discreteelectrodes in shapes of columns or pillars.

According to an embodiment, the discrete electrodes are arranged along asurface of a polyhedral or cylindrical tube or cone.

According to an embodiment, the focusing electrode comprises anelectrode in a shape of a polyhedral or cylindrical tube or cone.

According to an embodiment, the electrical contacts are at a firstsurface of the X-ray absorption layer; wherein the focusing electrodeextends between the first surface and a second surface of the X-rayabsorption layer opposite the first surface.

According to an embodiment, the focusing electrode is electricallyisolated from the electrical contacts.

According to an embodiment, the detector is further configured tocompile the added numbers as a spectrum of the X-ray photons incident onthe detector.

According to an embodiment, the plurality of pixels area arranged in anarray.

According to an embodiment, the pixels are configured to count thenumbers of X-ray photons within a same period of time.

According to an embodiment, each of the pixels comprises ananalog-to-digital converter (ADC) configured to digitize an analogsignal representing the energy of an incident X-ray photon into adigital signal.

According to an embodiment, the pixels are configured to operate inparallel.

According to an embodiment, the ADC is asuccessive-approximation-register (SAR) ADC.

According to an embodiment, the detector further comprises: a firstvoltage comparator configured to compare a voltage of the electricalcontact to a first threshold; a second voltage comparator configured tocompare the voltage to a second threshold; a controller; a plurality ofcounters each associated with a bin and configured to register a numberof X-ray photons absorbed by one of the pixels wherein the energy of theX-ray photons falls in the bin; wherein the controller is configured tostart a time delay from a time at which the first voltage comparatordetermines that an absolute value of the voltage equals or exceeds anabsolute value of the first threshold; wherein the controller isconfigured to determine whether an energy of an X-ray photon falls intothe bin; wherein the controller is configured to cause the numberregistered by the counter associated with the bin to increase by one.

According to an embodiment, the detector further comprises a capacitormodule electrically connected to the electrical contact, wherein thecapacitor module is configured to collect charge carriers from theelectrical contact.

According to an embodiment, the controller is configured to activate thesecond voltage comparator at a beginning or expiration of the timedelay.

According to an embodiment, the controller is configured to connect theelectrical contact to an electrical ground.

According to an embodiment, a rate of change of the voltage issubstantially zero at expiration of the time delay.

According to an embodiment, the X-ray absorption layer comprises adiode.

According to an embodiment, the X-ray absorption layer comprisessilicon, germanium, GaAs, CdTe, CdZnTe, or a combination thereof.

According to an embodiment, the detector does not comprise ascintillator.

Disclosed herein is a system comprising the detector described above andan X-ray source, wherein the system is configured to perform X-rayradiography on human chest or abdomen.

According to an embodiment, the system comprises the detector describedabove and an X-ray source, wherein the system is configured to performX-ray radiography on human mouth.

Disclosed herein is a cargo scanning or non-intrusive inspection (NII)system, comprising the detector described above and an X-ray source,wherein the cargo scanning or non-intrusive inspection (NII) system isconfigured to form an image using backscattered X-ray.

Disclosed herein is a cargo scanning or non-intrusive inspection (NII)system, comprising the detector described above and an X-ray source,wherein the cargo scanning or non-intrusive inspection (NII) system isconfigured to form an image using X-ray transmitted through an objectinspected.

Disclosed herein is a full-body scanner system comprising the detectordescribed above and an X-ray source.

Disclosed herein is an X-ray computed tomography (X-ray CT) systemcomprising the detector described above and an X-ray source.

Disclosed herein is an electron microscope comprising the detectordescribed above, an electron source and an electronic optical system.

Disclosed herein is a system comprising the detector described above,wherein the system is an X-ray telescope, or an X-ray microscopy, orwherein the system is configured to perform mammography, industrialdefect detection, microradiography, casting inspection, weld inspection,or digital subtraction angiography.

BRIEF DESCRIPTION OF FIGURES

FIG. 1A and FIG. 1B schematically show mechanisms of XRF.

FIG. 2 schematically shows a detector suitable for XRF, according to anembodiment.

FIG. 3 schematically shows a block diagram for the detector, accordingto an embodiment.

FIG. 4A schematically shows a cross-sectional view of the detector,according to an embodiment.

FIG. 4B schematically shows a detailed cross-sectional view of thedetector, according to an embodiment.

FIG. 4C schematically shows an alternative detailed cross-sectional viewof the detector, according to an embodiment.

FIGS. 5A and 5B each schematically show a cross-sectional view of adetector comprising focusing electrodes, according to an embodiment.

FIG. 6A and FIG. 6B each schematically show a top view of a focusingelectrode and electrical contacts, according to an embodiment.

FIG. 7A, FIG. 7B and FIG. 7C each schematically show a focusingelectrode, according to an embodiment.

FIG. 8A and FIG. 8B each show a component diagram of the electronicsystem of the detector, according to an embodiment.

FIG. 9 schematically shows a temporal change of the electric currentflowing through an electrical contact (upper curve) caused by chargecarriers generated by an X-ray photon incident on a pixel associatedwith the electrical contact, and a corresponding temporal change of thevoltage of the electrical contact (lower curve).

FIG. 10 schematically shows a system comprising the semiconductor X-raydetector described herein, suitable for medical imaging such as chestX-ray radiography, abdominal X-ray radiography, etc., according to anembodiment

FIG. 11 schematically shows a system comprising the semiconductor X-raydetector described herein suitable for dental X-ray radiography,according to an embodiment.

FIG. 12 schematically shows a cargo scanning or non-intrusive inspection(NII) system comprising the semiconductor X-ray detector describedherein, according to an embodiment.

FIG. 13 schematically shows another cargo scanning or non-intrusiveinspection (NII) system comprising the semiconductor X-ray detectordescribed herein, according to an embodiment.

FIG. 14 schematically shows a full-body scanner system comprising thesemiconductor X-ray detector described herein, according to anembodiment.

FIG. 15 schematically shows an X-ray computed tomography (X-ray CT)system comprising the semiconductor X-ray detector described herein,according to an embodiment.

FIG. 16 schematically shows an electron microscope comprising thesemiconductor X-ray detector described herein, according to anembodiment.

DETAILED DESCRIPTION

FIG. 2 schematically shows a detector 100 suitable for XRF, according toan embodiment. The detector has an array of pixels 150. The array may bea rectangular array, a honeycomb array, a hexagonal array or any othersuitable array. Each pixel 150 is configured to detect an X-ray photonincident thereon and measure the energy of the X-ray photon. Forexample, each pixel 150 is configured to count numbers of X-ray photonsincident thereon whose energy falls in a plurality of bins, within aperiod of time. All the pixels 150 may be configured to count thenumbers of X-ray photons incident thereon within a plurality of bins ofenergy within the same period of time. Each pixel 150 may have its ownanalog-to-digital converter (ADC) configured to digitize an analogsignal representing the energy of an incident X-ray photon into adigital signal. For XRF applications, an ADC with a 10-bit resolution orhigher is useful. Each pixel 150 may be configured to measure its darkcurrent, such as before or concurrently with each X-ray photon incidentthereon. Each pixel 150 may be configured to deduct the contribution ofthe dark current from the energy of the X-ray photon incident thereon.The pixels 150 may be configured to operate in parallel. For example,when one pixel 150 measures an incident X-ray photon, another pixel 150may be waiting for an X-ray photon to arrive. The pixels 150 may nothave to be individually addressable.

The detector 100 may have at least 100, 2500, 10000, or more pixels 150.The detector 100 may be configured to add the numbers of X-ray photonsfor the bins of the same energy range counted by all the pixels 150. Forexample, the detector 100 may add the numbers the pixels 150 stored in abin for energy from 70 KeV to 71 KeV, add the numbers the pixels 150stored in a bin for energy from 71 KeV to 72 KeV, and so on. Thedetector 100 may compile the added numbers for the bins as a spectrum ofthe X-ray photons incident on the detector 100.

FIG. 3 schematically shows a block diagram for the detector 100,according to an embodiment. Each pixel 150 may measure the energy 151 ofan X-ray photon incident thereon. The energy 151 of the X-ray photon isdigitized (e.g., by an ADC) in step 152 into one of a plurality of bins153A, 153B, 153C . . . . The bins 153A, 153B, 153C . . . each have acorresponding counter 154A, 154B and 154C, respectively. When the energy151 is allocated into a bin, the number stored in the correspondingcounter increases by one. The detector 100 may added the numbers storedin all the counters corresponding to bins for the same energy range inthe pixels 150. For example, the numbers stored in all the counters 154Cin all pixels 150 may be added and stored in a global counter 100C forthe same energy range. The numbers stored in all the global counters maybe compiled into an energy spectrum of the X-ray incident on thedetector 100.

FIG. 4A schematically shows a cross-sectional view of the detector 100,according to an embodiment. The detector 100 may include an X-rayabsorption layer 110 and an electronics layer 120 (e.g., an ASIC) forprocessing or analyzing electrical signals incident X-ray generates inthe X-ray absorption layer 110. In an embodiment, the detector 100 doesnot comprise a scintillator. The X-ray absorption layer 110 may includea semiconductor material such as, silicon, germanium, GaAs, CdTe,CdZnTe, or a combination thereof. The semiconductor may have a high massattenuation coefficient for the X-ray energy of interest.

As shown in a detailed cross-sectional view of the detector 100 in FIG.4B, according to an embodiment, the X-ray absorption layer 110 mayinclude one or more diodes (e.g., p-i-n or p-n) formed by a first dopedregion 111, one or more discrete regions 114 of a second doped region113. The second doped region 113 may be separated from the first dopedregion 111 by an optional the intrinsic region 112. The discrete regions114 are separated from one another by the first doped region 111 or theintrinsic region 112. The first doped region 111 and the second dopedregion 113 have opposite types of doping (e.g., region 111 is p-type andregion 113 is n-type, or region 111 is n-type and region 113 is p-type).In the example in FIG. 4B, each of the discrete regions 114 of thesecond doped region 113 forms a diode with the first doped region 111and the optional intrinsic region 112. Namely, in the example in FIG.4B, the X-ray absorption layer 110 has a plurality of diodes having thefirst doped region 111 as a shared electrode. The first doped region 111may also have discrete portions.

When an X-ray photon hits the X-ray absorption layer 110 includingdiodes, the X-ray photon may be absorbed and generate one or more chargecarriers by a number of mechanisms. An X-ray photon may generate 10 to100000 charge carriers. The charge carriers may drift to the electrodesof one of the diodes under an electric field. The field may be anexternal electric field. The electrical contact 119B may includediscrete portions each of which is in electrical contact with thediscrete regions 114. In an embodiment, the charge carriers may drift indirections such that the charge carriers generated by a single X-rayphoton are not substantially shared by two different discrete regions114 (“not substantially shared” here means less than 2%, less than 0.5%,less than 0.1%, or less than 0.01% of these charge carriers flow to adifferent one of the discrete regions 114 than the rest of the chargecarriers). Charge carriers generated by an X-ray photon incident aroundthe footprint of one of these discrete regions 114 are not substantiallyshared with another of these discrete regions 114. A pixel 150associated with a discrete region 114 may be an area around the discreteregion 114 in which substantially all (more than 98%, more than 99.5%,more than 99.9%, or more than 99.99% of) charge carriers generated by anX-ray photon incident therein flow to the discrete region 114. Namely,less than 2%, less than 1%, less than 0.1%, or less than 0.01% of thesecharge carriers flow beyond the pixel.

As shown in an alternative detailed cross-sectional view of the detector100 in FIG. 4C, according to an embodiment, the X-ray absorption layer110 may include a resistor of a semiconductor material such as, silicon,germanium, GaAs, CdTe, CdZnTe, or a combination thereof, but does notinclude a diode. The semiconductor may have a high mass attenuationcoefficient for the X-ray energy of interest.

When an X-ray photon hits the X-ray absorption layer 110 including aresistor but not diodes, it may be absorbed and generate one or morecharge carriers by a number of mechanisms. An X-ray photon may generate10 to 100000 charge carriers. The charge carriers may drift to theelectrical contacts 119A and 119B under an electric field. The field maybe an external electric field. The electrical contact 119B includesdiscrete portions. In an embodiment, the charge carriers may drift indirections such that the charge carriers generated by a single X-rayphoton are not substantially shared by two different discrete portionsof the electrical contact 119B (“not substantially shared” here meansless than 2%, less than 0.5%, less than 0.1%, or less than 0.01% ofthese charge carriers flow to a different one of the discrete portionsthan the rest of the charge carriers). Charge carriers generated by anX-ray photon incident around the footprint of one of these discreteportions of the electrical contact 119B are not substantially sharedwith another of these discrete portions of the electrical contact 119B.A pixel 150 associated with a discrete portion of the electrical contact119B may be an area around the discrete portion in which substantiallyall (more than 98%, more than 99.5%, more than 99.9% or more than 99.99%of) charge carriers generated by an X-ray photon incident therein flowto the discrete portion of the electrical contact 119B. Namely, lessthan 2%, less than 0.5%, less than 0.1%, or less than 0.01% of thesecharge carriers flow beyond the pixel associated with the one discreteportion of the electrical contact 119B.

The electronics layer 120 may include an electronic system 121 suitablefor processing or interpreting signals generated by X-ray photonsincident on the X-ray absorption layer 110. The electronic system 121may include an analog circuitry such as a filter network, amplifiers,integrators, and comparators, or a digital circuitry such as amicroprocessors, and memory. The electronic system 121 may includecomponents shared by the pixels or components dedicated to a singlepixel. For example, the electronic system 121 may include an amplifierdedicated to each pixel and a microprocessor shared among all thepixels. The electronic system 121 may be electrically connected to thepixels by vias 131. Space among the vias may be filled with a fillermaterial 130, which may increase the mechanical stability of theconnection of the electronics layer 120 to the X-ray absorption layer110. Other bonding techniques are possible to connect the electronicsystem 121 to the pixels without using vias.

FIG. 5A and FIG. 5B each schematically show a cross-sectional view of adetector comprising focusing electrodes, according to an embodiment.Each of the electrical contacts 119B may be surrounded by a focusingelectrode 1101. The focusing electrode 1101 comprises an electricallyconducting material such as polysilicon, a metal or a metal alloy. Theelectrical contacts 119B are at one of two opposite surfaces of theX-ray absorption layer; the focusing electrode extends between the twoopposite surfaces of the X-ray absorption layer.

During use, when an electric potential of the same polarity as thecharge carriers (e.g., negative potential if the electrical contacts119B collect electrons) the electrical contacts 119B collect from theX-ray absorption layer 110 is applied to the focusing electrode 1101,the electrical potential generates an electric field that directs to theelectrical contact 119B these charge carriers, which may be generated byan X-ray photon incident within confines of the focusing electrode 1101.The focusing electrode may be configured to restrict the movement ofthese charge carriers and direct these charge carriers to the electricalcontact 119B. The focusing electrode 1101 may be in a suitable form suchas a polyhedral or cylindrical tube or cone. According to an embodimentas shown in FIG. 5A, the focusing electrode 1101 is in the form of acylindrical tube. According to an embodiment as shown in FIG. 5B, thefocusing electrode 1101 is in the form of a cylindrical cone, with anarrower opening at an end closer to the electrical contacts 119B. Inanother embodiment, the focusing electrode 1101 may be in the form of aring or a series of rings. According to an embodiment, the focusingelectrode 1101 is not in direct contact with the electrical contacts119B and 119A, and is electrically isolated from the electrical contacts119B and 119A. According to an embodiment, the focusing electrode 1101is in direct contact with (and thus has the same electrical potentialas) the electrical contacts 119A, and is electrically isolated from theelectrical contacts 119B.

FIG. 6A and FIG. 6B each schematically show a top view of a focusingelectrode and electrical contact, according to an embodiment. Thefocusing electrode 1101 may be a polyhedral tube with two openings ofthe same size. A cross-sectional shape of the opening may be a square. Across-sectional area of the openings of the focusing electrode 1101 maybe larger than a cross-sectional area of the electrical contact 119B thefocusing electrode 1101 surrounds. Multiple focusing electrodes 1101 maybe arranged in an array so that each focusing electrodes 1101 surroundsone of the electrical contacts 119B. As shown in FIG. 6A, multiplefocusing electrodes 1101 may be merged. Alternatively, as shown in FIG.6B, multiple focusing electrodes 1101 may be arranged with a distance inbetween.

FIG. 7A, FIG. 7B and FIG. 7C each schematically show a focusingelectrode, according to an embodiment. FIG. 7A shows an example of thefocusing electrode 1101 in FIG. 6A which comprises a polyhedral tubewith two openings of square shapes. FIG. 7B shows an example of afocusing electrode 1106 which comprises a cylindrical tube with twoopenings of circle shapes. As mentioned above, the polyhedral tube andcylindrical tube such as those in FIGS. 7A and 7B may also be a coneshape with an opening away from the electrical contact 119B that isbigger than the opening proximal to the electrical contact 119B. FIG. 7Cshows an example of a focusing electrode 1107 which comprises discreteelectrodes in shapes of columns or pillars, and the discrete electrodesare arranged along a surface of a polyhedral or cylindrical tube orcone.

FIG. 8A and FIG. 8B each show a component diagram of the electronicsystem 121, according to an embodiment. The electronic system 121 mayinclude a first voltage comparator 301, a second voltage comparator 302,a plurality of counters 320 (including counters 320A, 320B, 320C, 320D .. . ), a switch 305, an ADC 306 and a controller 310.

The first voltage comparator 301 is configured to compare the voltage ofa discrete portion of the electrical contact 119B to a first threshold.The first voltage comparator 301 may be configured to monitor thevoltage directly, or calculate the voltage by integrating an electriccurrent flowing through the diode or electrical contact over a period oftime. The first voltage comparator 301 may be controllably activated ordeactivated by the controller 310. The first voltage comparator 301 maybe a continuous comparator. Namely, the first voltage comparator 301 maybe configured to be activated continuously, and monitor the voltagecontinuously. The first voltage comparator 301 configured as acontinuous comparator reduces the chance that the system 121 missessignals generated by an incident X-ray photon. The first voltagecomparator 301 configured as a continuous comparator is especiallysuitable when the incident X-ray intensity is relatively high. The firstvoltage comparator 301 may be a clocked comparator, which has thebenefit of lower power consumption. The first voltage comparator 301configured as a clocked comparator may cause the system 121 to misssignals generated by some incident X-ray photons. When the incidentX-ray intensity is low, the chance of missing an incident X-ray photonis low because the time interval between two successive photons isrelatively long. Therefore, the first voltage comparator 301 configuredas a clocked comparator is especially suitable when the incident X-rayintensity is relatively low. The first threshold may be 1-5%, 5-10%,10%-20%, 20-30%, 30-40% or 40-50% of the maximum voltage one incidentX-ray photon may generate on the electrical contact 119B. The maximumvoltage may depend on the energy of the incident X-ray photon (i.e., thewavelength of the incident X-ray), the material of the X-ray absorptionlayer 110, and other factors. For example, the first threshold may be 50mV, 100 mV, 150 mV, or 200 mV.

The second voltage comparator 302 is configured to compare the voltageto a second threshold. The second voltage comparator 302 may beconfigured to monitor the voltage directly, or calculate the voltage byintegrating an electric current flowing through the diode or theelectrical contact over a period of time. The second voltage comparator302 may be a continuous comparator. The second voltage comparator 302may be controllably activate or deactivated by the controller 310. Whenthe second voltage comparator 302 is deactivated, the power consumptionof the second voltage comparator 302 may be less than 1%, less than 5%,less than 10% or less than 20% of the power consumption when the secondvoltage comparator 302 is activated. The absolute value of the secondthreshold is greater than the absolute value of the first threshold. Asused herein, the term “absolute value” or “modulus” |x| of a real numberx is the non-negative value of x without regard to its sign. Namely,

${x} = \left\{ {\begin{matrix}{x,{{{if}\mspace{14mu} x} \geq 0}} \\{{- x},{{{if}\mspace{14mu} x} \leq 0}}\end{matrix}.} \right.$

The second threshold may be 200%-300% of the first threshold. Forexample, the second threshold may be 100 mV, 150 mV, 200 mV, 250 mV or300 mV. The second voltage comparator 302 and the first voltagecomparator 310 may be the same component. Namely, the system 121 mayhave one voltage comparator that can compare a voltage with twodifferent thresholds at different times.

The first voltage comparator 301 or the second voltage comparator 302may include one or more op-amps or any other suitable circuitry. Thefirst voltage comparator 301 or the second voltage comparator 302 mayhave a high speed to allow the system 121 to operate under a high fluxof incident X-ray. However, having a high speed is often at the cost ofpower consumption.

The counters 320 may be a software component (e.g., numbers stored in acomputer memory) or a hardware component (e.g., 4017 IC and 7490 IC).Each counter 320 is associated with a bin for an energy range. Forexample, counter 320A may be associated with a bin for 70-71 KeV,counter 320B may be associated with a bin for 71-72 KeV, counter 320Cmay be associated with a bin for 72-73 KeV, counter 320D may beassociated with a bin for 73-74 KeV. When the energy of an incidentX-ray photons is determined by the ADC 306 to be in the bin a counter320 is associated with, the number registered in the counter 320 isincreased by one.

The controller 310 may be a hardware component such as a microcontrollerand a microprocessor. The controller 310 is configured to start a timedelay from a time at which the first voltage comparator 301 determinesthat the absolute value of the voltage equals or exceeds the absolutevalue of the first threshold (e.g., the absolute value of the voltageincreases from below the absolute value of the first threshold to avalue equal to or above the absolute value of the first threshold). Theabsolute value is used here because the voltage may be negative orpositive, depending on whether the voltage of the cathode or the anodeof the diode or which electrical contact is used. The controller 310 maybe configured to keep deactivated the second voltage comparator 302, thecounter 320 and any other circuits the operation of the first voltagecomparator 301 does not require, before the time at which the firstvoltage comparator 301 determines that the absolute value of the voltageequals or exceeds the absolute value of the first threshold. The timedelay may expire after the voltage becomes stable, i.e., the rate ofchange of the voltage is substantially zero. The phase “the rate ofchange is substantially zero” means that temporal change is less than0.1%/ns. The phase “the rate of change is substantially non-zero” meansthat temporal change of the voltage is at least 0.1%/ns.

The controller 310 may be configured to activate the second voltagecomparator during (including the beginning and the expiration) the timedelay. In an embodiment, the controller 310 is configured to activatethe second voltage comparator at the beginning of the time delay. Theterm “activate” means causing the component to enter an operationalstate (e.g., by sending a signal such as a voltage pulse or a logiclevel, by providing power, etc.). The term “deactivate” means causingthe component to enter a non-operational state (e.g., by sending asignal such as a voltage pulse or a logic level, by cut off power,etc.). The operational state may have higher power consumption (e.g., 10times higher, 100 times higher, 1000 times higher) than thenon-operational state. The controller 310 itself may be deactivateduntil the output of the first voltage comparator 301 activates thecontroller 310 when the absolute value of the voltage equals or exceedsthe absolute value of the first threshold.

The controller 310 may be configured to cause the number registered byone of the counters 320 to increase by one, if, during the time delay,the second voltage comparator 302 determines that the absolute value ofthe voltage equals or exceeds the absolute value of the secondthreshold, and the energy of the X-ray photon falls in the binassociated with the counter 320.

The controller 310 may be configured to cause the ADC 306 to digitizethe voltage upon expiration of the time delay and determine based on thevoltage which bin the energy of the X-ray photon falls in.

The controller 310 may be configured to connect the electrical contact119B to an electrical ground, so as to reset the voltage and dischargeany charge carriers accumulated on the electrical contact 119B. In anembodiment, the electrical contact 119B is connected to an electricalground after the expiration of the time delay. In an embodiment, theelectrical contact 119B is connected to an electrical ground for afinite reset time period. The controller 310 may connect the electricalcontact 119B to the electrical ground by controlling the switch 305. Theswitch may be a transistor such as a field-effect transistor (FET).

In an embodiment, the system 121 has no analog filter network (e.g., aRC network). In an embodiment, the system 121 has no analog circuitry.

The ADC 306 may feed the voltage it measures to the controller 310 as ananalog or digital signal. The ADC may be asuccessive-approximation-register (SAR) ADC (also called successiveapproximation ADC). An SAR ADC digitizes an analog signal via a binarysearch through all possible quantization levels before finallyconverging upon a digital output for the analog signal. An SAR ADC mayhave four main subcircuits: a sample and hold circuit to acquire theinput voltage (V_(in)), an internal digital-analog converter (DAC)configured to supply an analog voltage comparator with an analog voltageequal to the digital code output of the successive approximationregister (SAR), the analog voltage comparator that compares V_(in) tothe output of the internal DAC and outputs the result of the comparisonto the SAR, the SAR configured to supply an approximate digital code ofV_(in) to the internal DAC. The SAR may be initialized so that the mostsignificant bit (MSB) is equal to a digital 1. This code is fed into theinternal DAC, which then supplies the analog equivalent of this digitalcode (V_(ref)/2) into the comparator for comparison with V_(in). If thisanalog voltage exceeds V_(in) the comparator causes the SAR to resetthis bit; otherwise, the bit is left a 1. Then the next bit of the SARis set to 1 and the same test is done, continuing this binary searchuntil every bit in the SAR has been tested. The resulting code is thedigital approximation of V_(in) and is finally output by the SAR at theend of the digitization.

The system 121 may include a capacitor module 309 electrically connectedto the electrical contact 119B, wherein the capacitor module isconfigured to collect charge carriers from the electrical contact 119B.The capacitor module can include a capacitor in the feedback path of anamplifier. The amplifier configured as such is called a capacitivetransimpedance amplifier (CTIA). CTIA has high dynamic range by keepingthe amplifier from saturating and improves the signal-to-noise ratio bylimiting the bandwidth in the signal path. Charge carriers from theelectrode accumulate on the capacitor over a period of time(“integration period”) (e.g., as shown in FIG. 9, between t_(S) to t₀).After the integration period has expired, the capacitor voltage issampled by the ADC 306 and then reset by a reset switch. The capacitormodule 309 can include a capacitor directly connected to the electricalcontact 119B.

FIG. 9 schematically shows a temporal change of the electric currentflowing through the electrical contact 119B (upper curve) caused bycharge carriers generated by an X-ray photon incident on the pixel 150associated with the electrical contact 119B, and a correspondingtemporal change of the voltage of the electrical contact 119B (lowercurve). The voltage may be an integral of the electric current withrespect to time. At time to, the X-ray photon hits the diode or theresistor, charge carriers start being generated in the pixel 150,electric current starts to flow through the electrical contact 119B, andthe absolute value of the voltage of the electrical contact 119B startsto increase. At time t₁, the first voltage comparator 301 determinesthat the absolute value of the voltage equals or exceeds the absolutevalue of the first threshold V1, and the controller 310 starts the timedelay TD1 and the controller 310 may deactivate the first voltagecomparator 301 at the beginning of TD1. If the controller 310 isdeactivated before t₁, the controller 310 is activated at t₁. DuringTD1, the controller 310 activates the second voltage comparator 302. Theterm “during” a time delay as used here means the beginning and theexpiration (i.e., the end) and any time in between. For example, thecontroller 310 may activate the second voltage comparator 302 at theexpiration of TD1. If during TD1, the second voltage comparator 302determines that the absolute value of the voltage equals or exceeds theabsolute value of the second threshold at time t₂, the controller 310waits for stabilization of the voltage to stabilize. The voltagestabilizes at time t_(e), when all charge carriers generated by theX-ray photon drift out of the X-ray absorption layer 110. At time t_(s),the time delay TD1 expires. At or after time t_(e), the controller 310causes the ADC 306 to digitize the voltage and determines which bin theenergy of the X-ray photons falls in. The controller 310 then causes thenumber registered by the counter 320 corresponding to the bin toincrease by one. In the example of FIG. 9, time t_(s) is after timet_(e); namely TD1 expires after all charge carriers generated by theX-ray photon drift out of the X-ray absorption layer 110. If time t_(e)cannot be easily measured, TD1 can be empirically chosen to allowsufficient time to collect essentially all charge carriers generated byan X-ray photon but not too long to risk have another incident X-rayphoton. Namely, TD1 can be empirically chosen so that time t_(s) isempirically after time t_(e). Time t_(s) is not necessarily after timet_(e) because the controller 310 may disregard TD1 once V2 is reachedand wait for time t_(e). The rate of change of the difference betweenthe voltage and the contribution to the voltage by the dark current isthus substantially zero at t_(e). The controller 310 may be configuredto deactivate the second voltage comparator 302 at expiration of TD1 orat t₂, or any time in between.

The voltage at time t_(e) is proportional to the amount of chargecarriers generated by the X-ray photon, which relates to the energy ofthe X-ray photon. The controller 310 may be configured to determine thebin the energy of the X-ray photon falls in, based on the output of theADC 306.

After TD1 expires or digitization by the ADC 306, whichever later, thecontroller 310 connects the electrical contact 119B to an electricground for a reset period RST to allow charge carriers accumulated onthe electrical contact 119B to flow to the ground and reset the voltage.After RST, the system 121 is ready to detect another incident X-rayphoton. Implicitly, the rate of incident X-ray photons the system 121can handle in the example of FIG. 9 is limited by 1/(TD1+RST). If thefirst voltage comparator 301 has been deactivated, the controller 310can activate it at any time before RST expires. If the controller 310has been deactivated, it may be activated before RST expires.

Because the detector 100 has many pixels 150 that may operate inparallel, the detector can handle much higher rate of incident X-rayphotons. This is because the rate of incidence on a particular pixel 150is 1/N of the rate of incidence on the entire array of pixels, where Nis the number of pixels.

FIG. 10 schematically shows a system comprising the semiconductor X-raydetector 100 described herein. The system may be used for medicalimaging such as chest X-ray radiography, abdominal X-ray radiography,etc. The system comprises an X-ray source 1201. X-ray emitted from theX-ray source 1201 penetrates an object 1202 (e.g., a human body partsuch as chest, limb, abdomen), is attenuated by different degrees by theinternal structures of the object 1202 (e.g., bones, muscle, fat andorgans, etc.), and is projected to the semiconductor X-ray detector 100.The semiconductor X-ray detector 100 forms an image by detecting theintensity distribution of the X-ray.

FIG. 11 schematically shows a system comprising the semiconductor X-raydetector 100 described herein. The system may be used for medicalimaging such as dental X-ray radiography. The system comprises an X-raysource 1301. X-ray emitted from the X-ray source 1301 penetrates anobject 1302 that is part of a mammal (e.g., human) mouth. The object1302 may include a maxilla bone, a palate bone, a tooth, the mandible,or the tongue. The X-ray is attenuated by different degrees by thedifferent structures of the object 1302 and is projected to thesemiconductor X-ray detector 100. The semiconductor X-ray detector 100forms an image by detecting the intensity distribution of the X-ray.Teeth absorb X-ray more than dental caries, infections, periodontalligament. The dosage of X-ray radiation received by a dental patient istypically small (around 0.150 mSv for a full mouth series).

FIG. 12 schematically shows a cargo scanning or non-intrusive inspection(NII) system comprising the semiconductor X-ray detector 100 describedherein. The system may be used for inspecting and identifying goods intransportation systems such as shipping containers, vehicles, ships,luggage, etc. The system comprises an X-ray source 1401. X-ray emittedfrom the X-ray source 1401 may backscatter from an object 1402 (e.g.,shipping containers, vehicles, ships, etc.) and be projected to thesemiconductor X-ray detector 100. Different internal structures of theobject 1402 may backscatter X-ray differently. The semiconductor X-raydetector 100 forms an image by detecting the intensity distribution ofthe backscattered X-ray and/or energies of the backscattered X-rayphotons.

FIG. 13 schematically shows another cargo scanning or non-intrusiveinspection (NII) system comprising the semiconductor X-ray detector 100described herein. The system may be used for luggage screening at publictransportation stations and airports. The system comprises an X-raysource 1501. X-ray emitted from the X-ray source 1501 may penetrate apiece of luggage 1502, be differently attenuated by the contents of theluggage, and projected to the semiconductor X-ray detector 100. Thesemiconductor X-ray detector 100 forms an image by detecting theintensity distribution of the transmitted X-ray. The system may revealcontents of luggage and identify items forbidden on publictransportation, such as firearms, narcotics, edged weapons, flammables.

FIG. 14 schematically shows a full-body scanner system comprising thesemiconductor X-ray detector 100 described herein. The full-body scannersystem may detect objects on a person's body for security screeningpurposes, without physically removing clothes or making physicalcontact. The full-body scanner system may be able to detect non-metalobjects. The full-body scanner system comprises an X-ray source 1601.X-ray emitted from the X-ray source 1601 may backscatter from a human1602 being screened and objects thereon, and be projected to thesemiconductor X-ray detector 100. The objects and the human body maybackscatter X-ray differently. The semiconductor X-ray detector 100forms an image by detecting the intensity distribution of thebackscattered X-ray. The semiconductor X-ray detector 100 and the X-raysource 1601 may be configured to scan the human in a linear orrotational direction.

FIG. 15 schematically shows an X-ray computed tomography (X-ray CT)system. The X-ray CT system uses computer-processed X-rays to producetomographic images (virtual “slices”) of specific areas of a scannedobject. The tomographic images may be used for diagnostic andtherapeutic purposes in various medical disciplines, or for flawdetection, failure analysis, metrology, assembly analysis and reverseengineering. The X-ray CT system comprises the semiconductor X-raydetector 100 described herein and an X-ray source 1701. Thesemiconductor X-ray detector 100 and the X-ray source 1701 may beconfigured to rotate synchronously along one or more circular or spiralpaths.

FIG. 16 schematically shows an electron microscope. The electronmicroscope comprises an electron source 1801 (also called an electrongun) that is configured to emit electrons. The electron source 1801 mayhave various emission mechanisms such as thermionic, photocathode, coldemission, or plasmas source. The emitted electrons pass through anelectronic optical system 1803, which may be configured to shape,accelerate, or focus the electrons. The electrons then reach a sample1802 and an image detector may form an image therefrom. The electronmicroscope may comprise the semiconductor X-ray detector 100 describedherein, for performing energy-dispersive X-ray spectroscopy (EDS). EDSis an analytical technique used for the elemental analysis or chemicalcharacterization of a sample. When the electrons incident on a sample,they cause emission of characteristic X-rays from the sample. Theincident electrons may excite an electron in an inner shell of an atomin the sample, ejecting it from the shell while creating an electronhole where the electron was. An electron from an outer, higher-energyshell then fills the hole, and the difference in energy between thehigher-energy shell and the lower energy shell may be released in theform of an X-ray. The number and energy of the X-rays emitted from thesample can be measured by the semiconductor X-ray detector 100.

The semiconductor X-ray detector 100 described here may have otherapplications such as in an X-ray telescope, X-ray mammography,industrial X-ray defect detection, X-ray microscopy or microradiography,X-ray casting inspection, X-ray non-destructive testing, X-ray weldinspection, X-ray digital subtraction angiography, etc. It may besuitable to use this semiconductor X-ray detector 100 in place of aphotographic plate, a photographic film, a PSP plate, an X-ray imageintensifier, a scintillator, or another semiconductor X-ray detector.

While various aspects and embodiments have been disclosed herein, otheraspects and embodiments will be apparent to those skilled in the art.The various aspects and embodiments disclosed herein are for purposes ofillustration and are not intended to be limiting, with the true scopeand spirit being indicated by the following claims.

What is claimed is:
 1. A detector, comprising: a plurality of pixels, each pixel configured to count numbers of X-ray photons incident thereon whose energy falls in a plurality of bins, within a period of time; an X-ray absorption layer; wherein the X-ray absorption layer comprises an electrical contact within each of the pixels, and a focusing electrode surrounding the electrical contact and configured to direct to the electrical contact charge carriers generated by an X-ray photon incident within confines of the focusing electrodes; and wherein the detector is configured to add the numbers of X-ray photons for the bins of the same energy range counted by all the pixels.
 2. The detector of claim 1, wherein the focusing electrode comprises polysilicon, a metal or a metal alloy.
 3. The detector of claim 1, wherein the focusing electrode is not in direct contact with the electrical contacts.
 4. The detector of claim 1, wherein the focusing electrode comprises discrete electrodes in shapes of columns or pillars.
 5. The detector of claim 4, wherein the discrete electrodes are arranged along a surface of a polyhedral or cylindrical tube or cone.
 6. The detector of claim 1, wherein the focusing electrode comprises an electrode in a shape of a polyhedral or cylindrical tube or cone.
 7. The detector of claim 1, wherein the electrical contacts are at a first surface of the X-ray absorption layer; wherein the focusing electrode extends between the first surface and a second surface of the X-ray absorption layer opposite the first surface.
 8. The detector of claim 1, wherein the focusing electrode is electrically isolated from the electrical contacts.
 9. The detector of claim 1, further configured to compile the added numbers as a spectrum of the X-ray photons incident on the detector.
 10. The detector of claim 1, wherein the plurality of pixels are arranged in an array.
 11. The detector of claim 1, wherein the pixels are configured to count the numbers of X-ray photons within a same period of time.
 12. The detector of claim 1, wherein each of the pixels comprises an analog-to-digital converter (ADC) configured to digitize an analog signal representing the energy of an incident X-ray photon into a digital signal.
 13. The detector of claim 1, wherein the pixels are configured to operate in parallel.
 14. The detector of claim 12, wherein the ADC is a successive-approximation-register (SAR) ADC.
 15. The detector of claim 1, further comprising: a first voltage comparator configured to compare a voltage of the electrical contact to a first threshold; a second voltage comparator configured to compare the voltage to a second threshold; a controller; a plurality of counters each associated with a bin and configured to register a number of X-ray photons absorbed by one of the pixels wherein the energy of the X-ray photons falls in the bin; wherein the controller is configured to start a time delay from a time at which the first voltage comparator determines that an absolute value of the voltage equals or exceeds an absolute value of the first threshold; wherein the controller is configured to determine whether an energy of an X-ray photon falls into the bin; wherein the controller is configured to cause the number registered by the counter associated with the bin to increase by one.
 16. The detector of claim 15, further comprising a capacitor module electrically connected to the electrical contact, wherein the capacitor module is configured to collect charge carriers from the electrical contact.
 17. The detector of claim 15, wherein the controller is configured to activate the second voltage comparator at a beginning or expiration of the time delay.
 18. The detector of claim 15, wherein the controller is configured to connect the electrical contact to an electrical ground.
 19. The detector of claim 15, wherein a rate of change of the voltage is substantially zero at expiration of the time delay.
 20. The detector of claim 15, wherein the X-ray absorption layer comprises a diode.
 21. The detector of claim 15, wherein the X-ray absorption layer comprises silicon, germanium, GaAs, CdTe, CdZnTe, or a combination thereof.
 22. The detector of claim 1, wherein the detector does not comprise a scintillator.
 23. A system comprising the detector of claim 1 and an X-ray source, wherein the system is configured to perform X-ray radiography on human chest or abdomen. 